1. Technical Field
The present invention relates to a boiling water reactor having a reactor pressure vessel equipped with an openable upper lid.
2. Background Art
A typically known boiling water reactor will be described below by referring to FIG. 5. FIG. 5 is a schematic diagram of a known boiling water reactor, showing a cross-sectional elevation of a principal part of the reactor pressure vessel and the system configuration of the vent line and the head spray line thereof.
The vent line 21 of the reactor pressure vessel 1 of a conventional nuclear power plant is installed to exhaust gas and steam from or supply gas and steam to the inside of the reactor pressure vessel 1 when the nuclear reactor is shut down, and penetrates a reactor pressure vessel upper lid 2. The vent line 21 arranged at a top part of the reactor pressure vessel 1 includes a non-condensable gas exhaust line 23 arranged to exhaust non-condensable gas that can be accumulated in the top part of the reactor pressure vessel 1 while the nuclear reactor is operating. An end of the vent pipe is connected to a main steam pipe 20.
A head spray line 22 is branched from the vent line 21 as disclosed in Japanese Patent Application Laid-Open Publication No. 11-231088, the entire content of which is incorporated herein by reference. When the nuclear reactor is shut down, the steam discharged from a dryer 26 can be cooled by spraying water into the gas phase section in the reactor pressure vessel upper lid 2 by way of the head spray line 22.
Additionally, the water level in the inside of the reactor pressure vessel 1 needs to be raised and lowered when the nuclear reactor is started and shut down, and a reactor water level gauge 7 is provided to monitor the water level. An upper take-out point of the reactor water level gauge 7 is branched out from the top part of the reactor pressure vessel 1.
The fuel in the reactor core 34 in the reactor pressure vessel 1 keeps on emitting decay heat after the nuclear reactor is shut down. In order to remove the decay heat, the reactor water in the reactor pressure vessel 1 is partly taken out, pressurized by a reactor water circulation pump 16 and cooled by way of a reactor water cooling heat exchanger 17. Then, the cooled water is returned to the inside of the reactor pressure vessel 1.
Besides, water can be fed to the head spray line 22 from the downstream of the reactor water cooling heat exchanger 17. More specifically, steam can be cooled in the reactor pressure vessel 1 by partly utilizing the water to be returned to the reactor pressure vessel 1 so as to spray the water into the gas phase section in the reactor pressure vessel upper lid 2.
The piping of the reactor pressure vessel vent line 21 and the head spray line 22 is disposed on the heat insulating material 15 of the reactor pressure vessel 1. Therefore, the heat insulating material 15 cannot be removed unless the reactor pressure vessel flanges 24 are uncoupled and the piping on the reactor pressure vessel upper lid 2 is taken away.
It is required to reduce the shut down period of the nuclear reactor of a nuclear power plant in order to improve the operation rate of the power plant. In recent years, the head spray line has been employed to cool the upper part of the nuclear reactor in order to improve the cooling rate of the nuclear reactor and to open the reactor pressure vessel as soon as possible.
When the nuclear reactor is shut down and the temperature of the reactor water is 100 degrees Celsius or higher, the piping connected to the reactor pressure vessel 1 cannot be removed because the inside of the reactor pressure vessel upper lid 2 is filled with steam. The heat insulating material 15 of the reactor pressure vessel 1 cannot be removed in such a condition.
Therefore, in order to open the reactor pressure vessel 1 early, it is desirable to remove the reactor pressure vessel heat insulating material 15 early and, when the reactor water temperature falls well below 100 degrees Celsius, quickly take away the reactor pressure vessel upper lid 2.
To take away the reactor pressure vessel upper lid 2, the operation of loosening the stud bolts 11 tightly binding the reactor pressure vessel flanges 5 together needs to be conducted quickly. However, since the head spray line 22 is not so designed as to directly spray water to the inner wall of the reactor pressure vessel 1, it takes time to cool the reactor pressure vessel 1 even by using the head spray line 22.